In a mixing bowl, cream together butter or margarine until fluffy. Gradually add sugar, beating well after each addition until the mixture is fluffy. Add eggs, one at a time, and beat well after each addition. Stir in lemon rind and lemon juice. In a separate bowl, sift together flour, baking powder, and salt, and then stir it into the first mixture, stirring only until blended. Spoon the batter into a prepared pan and bake for approximately 1 hour and 15 minutes, or until a toothpick inserted into the center of the cake comes out clean. Turn the cake out onto a rack to cool.